1. Field of the Invention
The present invention relates generally to helix antennas and, more particularly, to methods of feeding and matching bifilar and/or quadrifilar slotted helix antennas. In one possible embodiment, a pipe of cables passes through the slotted helix antenna.
2. Description of the Prior Art
A bifilar helix antenna comprises two antenna elements. A quadrifilar helix antenna effectively has two bifilar antennas spaced ninety degrees apart from each other about the antenna axis.
U.S. Pat. No. 4,349,824 to E. Harris, issued Sep. 14, 1982, discloses an around-a-mast quadrifilar microstrip antenna where a down link UHF antenna is designed which includes four equally spaced arms plated at an angle on a fiberglass cylinder. A coaxial connector is connected to each of the four arms with the metal plated inner surface as the common ground. This helix has an inner metal cylinder, which serves as the metal plated inner surface described above, inserted up its axis to allow cables and other functions to pass the antenna. This forces the normal on axis quadrature feed point to be spread out symmetrically to at least the diameter of the inner cylinder. The presence of the inner cylinder symmetrically placed within the antenna allows the feed end of each element to be referenced to the common cylinder, so that that the feed end of each element can be fed via the center conductor of a coaxial cable, whose outer conductor is connected to the inside of the common cylinder. The common cylinder can be referenced as a ground plane only if it is large enough, to the extent where coupling between an element and the cylinder dominates coupling between an element and its radially opposite corresponding bifilar element. The antenna is fed via four coaxial cables that feed in phase quadrature, all referenced to the common cylinder. Placement of an inner cylinder on the antenna axis reduces the bandwidth of the antenna. Bandwidth decreases with the diameter of the inner cylinder. Thus, the inner cylinder should be of minimum diameter.
Normally, a prior art helix antenna is wrapped about a cylinder axially centered on the antenna axis, for a certain number of turns or pitch angle.
In the feed region, the two elements of a bifilar antenna bend radially and taper in width towards the axis to two points, which become the feed point of the helix. The bent and tapered section of the element may be referred to as a feed arm. The two feed points are fed one hundred eighty degrees apart in phase. For the quadrifilar helix antenna, ninety degrees is added or subtracted to the feed phase of the second bifilar pair, depending upon which end of the helix the unidirectional pattern is to radiate from.
A typical impedance locus of a bifilar helix antenna when the total element length is ¼ wavelength and the diameter is 0.1 to 0.2 wavelengths starts as an open circuit impedance at zero frequency, and spirals quickly with frequency to and through a low resonance resistance of R0. Typical values of R0 are in the range of 5 to 10 ohms. Other impedance resonances are also possible, such as ½ wavelength shorted, and ¾ wavelength open. They are similar to the ¼ wavelength open impedance, except for the fact that they have higher R0 values.
The bandwidth of the helix is reduced some by the indirect path of an antenna element from the feed point to the end thereof. This is largely due to the bend where the radial sections of the antenna elements begin. Maximum bandwidth occurs with a minimum path length (straight path) from the feed point to the element end, while maintaining the antenna radius/volume. Any extra path length adds inductance to the impedance of the element.
A shorted end fire slot antenna typically has an exponential taper of the antenna elements. Beyond the feed point, the antenna is open where most of the radiation occurs. Before the feed point, is a shorted inductive section. Since the shorted section is implemented on the antenna, a small radiation is associated with it. If the shorted section becomes narrow enough, it essentially becomes an inductance with no radiation loss and thus can be replaced by a simple inductance. Radiation from the shorted section will improve overall antenna impedance bandwidth.
Two possible geometries are utilized in the shorted section. In one geometry, a common elongated path realizes an inductance and an alternative geometry where the slot is simply continued and then shorted. The shorted section is an inductance that matches the antenna. The inductive short wraps the low frequency part of the resonance locus from near an open impedance to a short, introducing a loop, which can increase bandwidth over the bandwidth of the frequencies of the loop. At some point, part or all of the locus loop wraps about or is centered about a higher R0 (which may preferably be 50 ohms), which is partially determined by the width and the taper of the slot. The locus is tighter and has more bandwidth for wider antennas because radiation also starts to occur along the width dimension perpendicular to the slot of the antenna.
An open end fire slot is combined with a flared bowtie antenna to make a hybrid antenna called the Slotline/Bowtie Hybrid Antenna. From the feed end of the slot, the antenna flares out in a bowtie shape in the cylindrical space square to the slot surface and parallel to the slot axis. The slot edge becomes two dimensional, thus increasing the possible bandwidth of the antenna. The antenna has large pattern and impedance bandwidth.
In certain prior art quadrifilar helix antennas, the four antenna elements taper radially (radial feed arm sections of elements) from a quadrature feed point and then bend over the end of a support tube to continue down the length of the tube as a helix. On certain of these antennas, the antenna elements are metal tubes. To allow cables to pass the antenna, a sheath of cables passes by most of the antenna by existing on the antenna axis, which is an rf=0 point. At the feed point, the sheath of cables bends around the feed point between two radial feed arms, and then continues along the antenna axis. Although the bend introduces an asymmetry in the geometry, it only has a small effect on antenna impedance and pattern symmetry.
The present invention provides an alternate method of feeding and matching a bifilar or quadrifilar helix antenna. In one embodiment of the invention, a pipe of cables passes the antenna.